To increase the power flow capacity on transmission lines within an electric power system, a series capacitor bank may be connected in series with the transmission lines. The series capacitor bank may have negative interactions with other components within the power system connected to the transmission lines.
The other components may include torsional systems of power generators, and the incorporation of a series capacitor bank may destabilize the torsional systems of the power generators, thereby causing high-magnitude, short-duration transient torques (also known as “transient torque amplification”) and/or unstable/growing oscillatory torques (also known as “sub-synchronous resonance” or SSR) which can cause damage to the mechanical torsional systems (shafts, couplings, bearings) of power generators.
The series capacitors can also interact with electrical and/or control-system components, causing electrical instability (also known as “sub-synchronous control interaction” or SSCI), e.g., high-magnitude current flows and voltage swings, thereby damaging any of the electrical components within the path of that interaction.
Further, power swing oscillation may occur when two strong power systems are connected through a weak transmission connection such that if a large disturbance occurs e.g., a fault, the two strong power systems may exchange large amounts of real and reactive power in low-frequency oscillations that can be high-magnitude and/or unstable/growing, thereby causing large current and voltage swings which can damage any of the electrical components within the path of that power swing oscillation.
Two devices have been used in the past to damp out, restrict, or eliminate some or all of the above-mentioned types of oscillations which are damaging to the power systems. These two devices are known as a passive damping filter and a thyristor-controlled series capacitor (TCSC). FIG. 1 is a schematic of a conventional series capacitor including a passive damping filter circuit, and FIG. 2 is a schematic illustration of a conventional TCSC.
In FIG. 1, a main capacitor bank 110 is in series connection with the transmission line 100 and a damping filter circuit 120 is placed in parallel with the main capacitor bank 110, and includes a filter resistor 122, a filter capacitor 124 and inductor filter reactor 126. The filter resistor 122 damps the sub-synchronous series resonance caused by the main capacitor bank 110. The filter capacitor 124 and the filter reactor 126 are connected in parallel, and in series with the resistor 122.
The capacitor 124 and the filter reactor 126 block current in resistor 122 at the synchronous frequency in order to minimize steady-state conduction and thus power dissipation in the resistor 122. When there are sub-synchronous currents, i.e. currents whose frequency is less than power frequency, flowing through the transmission line, the sub-synchronous portion of currents will be diverted away from the main capacitor 110 and filter capacitor 124.
Instead, the sub-synchronous portion of currents will be directed towards the filter resistor 122 and filter reactor 126 path, providing broad resistive damping to increase electrical and torsional stability. This redirection also provides a resistive path for stored energy in the main capacitor 110 to dissipate without causing high transient torques across the electrical air gap of a power generator. This circuit does not affect or otherwise mitigate the phenomenon of power oscillation damping.
In FIG. 2, the conventional TCSC includes a main capacitor 210 is in series connection with a transmission line 200, and a thyristor-controlled reactor circuit 220 that includes a thyristor valve reactor 220 serially connected with a thyristor valve 230 having a pair of anti-parallel connected thyristors which operate as a switching device. The thyristor valve 230 is used to control the conduction duration of the thyristor valve reactor 220 and thereby control, within a range, the effective series reactance of the TCSC bank.
A continuous control of TCSC reactance is also known as Vernier operation and may, depending on the specific controls used, act to provide positive/stabilizing damping on the torsional, electrical, and power oscillation stability issues mentioned before. Active control of a conventional TCSC is performed by a controller with many possible functions. This multiplicity of functionality may cause short periods of time when conflicting control commands lead to a prioritization and a selection of a control mode that is beneficial for one function and detrimental to another.
For example, the function of power oscillation damping may cause the thyristor valve therein to block conduction entirely, and when blocked, it creates an open circuit leaving the main capacitor 210 as a simple fixed series capacitor bank, and is therefore not performing the SSR mitigation function. Because of this and other issues, the conventional TCSC cannot be relied upon to mitigate the phenomenon of transient torque amplification.
Another issue associated with the TCSC, is that in order to perform certain mitigations effectively, the thyristor valve therein is in continuous Vernier operation which requires the TCSC components e.g., the thyristor valve reactor 222 and the thyristor valve 224 to be rated for steady state current, and while conducting in a steady state, the thyristor valve generates heat and in order to remove the heat and avoid damage to the thyristor components, deionized water is passed through a heat exchanger to pump the deionized water up to the high voltages of transmission lines, which can range from 138 kV to as high as 1000 kV.
Since the capacitor banks are connected in series with the high voltage transmission lines, the deionized water needs to be pumped to a high voltage comparable to the high voltage on the transmission lines.
These two types of conventional series capacitors, with passive damping filter circuits 120 or TCSC components, are used for various transmission system functions including, for example, sub-synchronous resonance (SSR, torsional stability) mitigation, sub-synchronous control interaction (SSCI, electrical stability) mitigation, transient torque amplification mitigation, power swing oscillation damping, and power flow control.